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HDGene, ASE Certified Technician
Category: Lincoln
Satisfied Customers: 5574
Experience:  25 year ASE Master Technician & Advanced Drivability L1 Certified Ford Senior Master Technician
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I have a 1993 Lincoln town car. The hvac works as designed

Customer Question

I have a 1993 Lincoln town car. The hvac works as designed for about 5 minutes of driving. After that, it gets stuck on defrost mode and won't change to vent, floor or respond at all. Sometimes when I turn the hvac off then on, it will work momentarily and get stuck on defrost again. What is wrong with this system?
Submitted: 11 months ago.
Category: Lincoln
Expert:  HDGene replied 11 months ago.

Hello, and thank you for using Just Answer! My name is***** me a few moments to review your question and Ill do my best to assist you.

Expert:  HDGene replied 11 months ago.

Hello, is this an EATC digital climate control system? Does it only change to defrost mode while driving or will it do it at idle as well?

Customer: replied 11 months ago.
the hvac is the electronic automatic temperature control. It seems to only get stuck on defrost mode and stay there when driving with heavier throttle. It blows through the vents fine at idle. But once it gets stuck on defrost, it does not operate properly at idle or cruising.
Expert:  HDGene replied 11 months ago.

Ok if it only happens while driving and then will revert back to normal operation it will be an issue with the vacuum supply to the climate control head. Either a leak in the vacuum supply, an issue with the check valve or crack in the vacuum resouvoir tank. You can verify this by removing the control head and testing the vacuum supply to the control head. If it looses vacuum, there is a leak int he supply, if it doesnt loose vacuum, then the issue is in the climate control head itself.

Customer: replied 11 months ago.
I will try to remove the control head and test the vacuum supply. Is there a specific amount of vacuum it should have or I am just checking to see if it holds vacuum? How do you remove the surrounding trim to remove the control head? Are you able to provide a schematic on the system for the car? I hope the vacuum supply line, reservoir and check valve aren't too hard to access. Thanks a lot.
Expert:  HDGene replied 11 months ago.

Engine supply, apprx 17- 19 inches of steady vacuum. The check valve is supposed to seal off the vacuum tank because when you go wide open throttle you loose all engine vacuum supply.

Customer: replied 11 months ago.
Where is the vacuum canister?
Expert:  HDGene replied 11 months ago.

It should be located under the dash below the glove box area. The vacuum supply will come from under the hood by the ac evaporator case through the firewall to the under glove box area. The round black plastic tank connects to the supply line and runs to the vacuum black source line to the control head. So you can trace it back either way.

Customer: replied 11 months ago.
I am going to get this checked out in the morning. Thank you for very much.
Expert:  HDGene replied 11 months ago.

You are welcome, if you need more help, let me know. If I was helpful, please leave a positive rating.

Customer: replied 11 months ago.
Alright. I hooked up a vacuum gauge inline with the source vacuum right after the check valve located on the firewall. I road tested the car with the guage taped to the windshield. Right when the gauge dips below 15 in hg the defrost only blows and it doesn't change until you shut the eatc off and back on when vacuum is around 20 in hg. The vacuum canister is located on the driver side wheel liner next to the relay center. I tested vacuum and pumped it to 25 in hg. It dipped to approx 15 in hg after a half hour. I think the check valve is the problem. I am going to test it next once the engine cools off. Does this sound like proper diagnosis to you? I think I am close. Thanks for all your advise.
Expert:  HDGene replied 11 months ago.

Yes the check valve is supposed to hold pressure, so if it bleeds off, then this is what I would replace first. Pay attention to how it is installed, if you install the new one backwards it wont work correctly.

Expert:  HDGene replied 11 months ago.

Do you need more help with this concern?